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V.7.2(July, 77)7.2 DESIGN PRINCIPLES7.2.1 Pesign parametersThe following parameters should be considered:a the design traffic;the design life;(c the strength of the subgradej and f(d) the type of pavement *Bstimation of design traffioTraffic damage to highway pavements increases progressivelywith the weight of individual axle loads* The loads imposed byprivate cars cause no significant damage to pavements, whereasaxle loads of the maximum permitted weight have very considerabledamaging power, and in general only the loads imposed by commercialvehicles need to be considered for pavement design purposes* Inthis context, a commercial vehicle is a goods or public servicevehicle of unladen weight exceeding 1500 kg* The axle loadsimposed by commercial vehicles vary widely and for design purposestheir effects may be represented by equivalent numbers of standardaxles of 8200 kg each* The actual factor used in pavement designis the cumulative number of standard axles passing during thedesign life of the pavement* This is derived in three stages:(i) the number of commercial vehicles using the pavementimmediately after completion is first estimated;(ii)(iii)a growth rate is then assumed and applied to obtainthe cumulative number of vehicles passing during thedesign life of the pavement;a co—efficient based on the expected characteristicsof the traffic is finally applied to convert thenumber of commercial vehicles to the number ofstandard axles passing during the design life ofthe pavement»Figures ?*?*! to Tf»7*4 graphically relate cumulative totalsof commercial vehicles passing to initial traffic and design lifefor growth rates of 3$ t 4^f S$* and 6$ per annum*Alternatively, the expressionA -. P (l+r) nwhere A « the number of commercial vehiclesper day for the f n f th year: